<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width">
    <title>repl.it</title>
    <link href="style.css" rel="stylesheet" type="text/css" />
  </head>
  <body>
     <input type="text" id="one">
     <select>
      <option>请选择</option>
      <option value="add">+</option>
      <option value="minus">-</option>
      <option value="multiply">*</option>
      <option value="divide">/</option>
    </select>
    <input type="text" id="two">
    <input type="button" value="=" id="btn">
    <input type="text" id="three">
  
    <script src="https://cdn.staticfile.org/jquery/1.10.2/jquery.min.js"></script>
    <script>
      var $one = $("#one")
      var $two = $("#two")
      var $three = $("#three")
      var $btn = $("#btn")
      var $se = $("select")
      var $opt = $("option")
      var $res
      $btn.click(function () {
        switch ($se.val()) {
            case 'add':
              $res = (parseFloat($one.val()) + parseFloat($two.val()));
              break;
            case 'minus':
              $res = (parseFloat($one.val()) - parseFloat($two.val()));
              break;
            case 'multiply':
              $res = (parseFloat($one.val()) * parseFloat($two.val()));
              break;
            case 'divide':
              $res = (parseFloat($one.val()) / parseFloat($two.val()));
              break;
            default:
              $res =`Sorry, wrong input.`;
              break;
          }
        $three.val($res)
       
      })
      
    </script>
  </body>
</html>